Opiate receptors recognize endogenous opioid peptide neurotransmitters and exogenous opiate drugs of high abuse liability and analgesic benefits. Opiate receptors produce their effects in cells by altering intracellular G-protein linked second messengers and altering opening of G-protein-linked channels, especially those conducting potassium and calcium. In order to improve understanding these key elements in mediating opiate addictions, we have used cloned, mutant, deleted, and chimeric opiate receptor cDNAs to identify portions of the morphine-preferring mu opiate receptor that are important for coupling to second messenger systems. Molecular cloning studies have identified cDNAs encoding each of the principal pharmacologically defined opiate receptors from several species. Expression of these cDNAs and their mutants in COS cells has revealed striking differences in coupling to G-protein-linked second messenger systems. Both human and rat muORs can mediate morphine and DAMGO inhibition of forskolin-stimulated adenyl cyclase activity in COS and/or CHO cell expression systems. In addition, morphine stimulates IP3 accumulation in cells expressing hmuOR. Mutagenesis has revealed the importance of charged transmembrane domain residues and the C-terminal 33 amino acids of the rmuOR for the intrinsic activity of the receptor. These results underscore the specificity of second messenger effects that can be observed in COS cell systems with members of this gene family.